1. Field of the Invention
The present invention relates to the field of heating apparatuses for microdevices, the method of fabricating such heating apparatuses and their methods of use, and more particularly to processes, structures and materials for the construction and use of ink-jet print heads, other microinjection devices, microelectromechanical devices and chemical analysis devices.
2. Description of the Related Art
A very common device in use today is the ink-jet print head. Ink-jet printers are superior to dot matrix printers, being able to print in multiple colors, with less noise and with better print quality.
The thermal ink-jet print head is a specific example of a structure that is representative of the class of microinjection devices, which are devices which expel small, controlled amounts of a liquid, thereby injecting the liquid into the target. In general, the ink-jet print head has a plurality of discrete micro-injectors, formed in an array, each with an orifice, or nozzle, of small diameter. Upon receiving an electrical signal, the electricity is used to heat a liquid to expand or vaporize it, expelling ink through the nozzle and onto the paper.
An exemplary ink-jet print head generally contains a heater section in which a heater resistor layer is formed on a substrate and an electrode layer is formed on the heater resistor layer to provide electrical contact. This heater section heats a working fluid which vaporizes, expanding a membrane which drives the expulsion of an ink drop. I have noticed that this ink-jet printer head design, however, is subject to several problems. First, the heater resistor layer and electrode layer are generally made of different materials, and adhesion between these layers can be weak. Chemical reactions occurring in the etching process used to pattern the layers can lead to gradual deterioration of the adhesion zone between the two layers. As a result, a gap can form between the layers. Secondly, during use, the working liquid contacting these layers may seep into the gap between the two layers, causing further deterioration. Thirdly, the mechanical stress caused by the vibration of the membrane can also cause deterioration of the contact between the layers. When such a gap forms, it leads to irregularities in the vapor pressure of the working liquid. This in turn causes irregularities in the vibration of the membrane and leads to poor formation of the ink drop and thus poor performance of the print head.
An example of earlier efforts to address a related problem is U.S. Pat. No. 5,223,855, to Ota et al., entitled Thermal Head For A Printer. This deals with the deformation of a thermal head due to the difference in thermal expansion between members and describes use of a soft adhesive to adhere the substrate containing the heating elements to a support board. This use of adhesive does not solve, however, the problem of adhesion of the electrodes to the heating resistor.
I have observed that what is needed, then, is a method for insuring that the adhesion of the electrodes to the heater resistor is robust, and has a long lifetime in devices such as ink-jet print heads.